Scout was a towel boy, I mean, assistant manager, for the college baseball team. Then one day they were down a man, and sent him out to play. The instant he was given that bat, he just lost his head, or, everyone else lost THEIR heads.
Soldier went to a school in Russia and blew it up.
Pyro was a physics teacher who fell in love with a chemistry teacher, and then he went crazy after a freak napalm experiment.
Demoman never went to college, his parents died and he had no way of funding for college.
The Heavy went to a cooking class. How else do you think he makes such delicious sandviches? For goodness sake the sheer taste of them alone heals bullet wounds.
Sniper never went to college, he lived on the open road, but picked up things in his travels.
Medic was a mortitioner, and he quit because the bodies didn't squirm.
The Spy has gone to many schools impersonating a teacher, a student, a janitor, a chair, a pencil, and a projector that somehow worked.